Plotting LOPs eight miles high

Jan 1, 2003

You might think that, in today's world of GPS and inertial navigation, the techniques of plotting LOPs and determining running fixes are a lost art. However, some of these plotting techniques remain in the cockpit manuals of a few of our nation's airlines, which must still verify the accuracy of those little black boxes on extended flights over the vast oceans of the world, where ground-based navigation aids are scarce.

Like most other airlines, the fleet of Northwest Airlines, the flag carrier I fly for, has a mixed bag of aircraft type and age. Although upgrades to ring-laser gyro-based inertial navigation systems (INS) and GPS updating capability will soon find their way onto our andquot;genericandquot; B-747 models (the older three-pilot aircraft that Northwest flies in addition to its new generation B-747-400 series), we still use the older Delco Carousel gyro INS. This unit utilizes spinning mechanical gyros that sense acceleration in a two-dimensional plane. After its present position is loaded into its memory, and the unit has aligned itself with true north, the INS will read out its position, speed, heading, course, and even wind direction and speed. These INSs happen to be the same units installed in the military transport aircraft I also fly for the Air Force Reserve, the Lockheed C-141B Starlifter. They are 1970s technology, and they have worked very well for the commercial and military world. However, the acceptable error rate of 2.5 nm/hour at times does not yield the kind of accuracy needed for today's crowded skies. This is definitely a factor on the long 12- to 14-hour Pacific crossings. Even with three INSs talking to each other to determine the present mean latitude and longitude, if more than one of the triple-mixed units drift off course, even within the 2.5 nm/hour tolerance, the Air Traffic Control of most nations, including the U.S., will label the incident as an unacceptable off-course deviation, or violation.

Although the navigator's position on most airlines and military transports was eliminated with the advent of inertial navigation, the pilots must still perform some verification functions to detect the drifting of the older nav units and apply the proper correction to keep the flight on its filed flight path. This is usually accomplished using ground-based VOR/TACAN (VORTAC) stations, which give on-board instruments magnetic radial and distance from the station. These bearing and distances can be plotted to check the INS units as you andquot;coast-outandquot; over water, and andquot;coast-inandquot; to your destination. However, using whatever navigation fixes you can find en route will greatly increase the chances of determining which navigation unit is drifting, if their three readouts of inertial position begin to differ significantly from one another.

This year, Northwest is celebrating its 50th year of flying the Pacific, and a few of its old navigation plotting techniques are still taught and used on over-water flights. As a Honolulu-based B-747 pilot, I have the pleasure of plotting running LOPs using information from distance-measuring equipment (DME) on Midway Island, which usually lies near to the flight path between Honolulu and Tokyo, one of our busiest routes.

A C-141 military transport can receive distance and bearing from the solitary TACAN station on Midway, and we can even fly an approach to the small runway using only the data received from the TACAN, assuming we can successfully dodge the thousands of gooney birds that try their best to impale themselves on various parts of the aircraft. However, civilian VOR receivers cannot interpret the bearings from military TACANs, so a running LOP, and/or a radar fix, if available, must be plotted. The radar used on board large transport aircraft are mainly designed to detect thunderstorms, and their mapping mode, at times, may have difficulty tuning out weather and wave returns. Also, the bearing and range information obtained from an aircraft weather radar is only accurate to about five to 10 miles, and 10 to 15 degrees, given the altitude and high speed of the aircraft and the fuzzy returns used for plotting.

Our flight operations manual gives detailed instruction on how to plot the running LOP, which is supposedly an art only associated with maritime celestial navigation. Although easy to do, and very basic in theory, it does yield a somewhat accurate assessment of where you are in relation to your plotted course line.

As soon as we begin receiving distance from Midway (usually about 180 nautical miles away) we freeze the position of one of the INS units, plot it, and record the DME from Midway and the miles to go (MTG) to the next waypoint, usually an even latitude and longitude. The DME is plotted as an arc with arrowheads and is labeled appropriately. Then, about 20 minutes and a couple hundred nautical miles later, another INS plot is taken, with its corresponding DME and MTG. This plot's MTG is subtracted from the first, which yields the distance traveled between plots. The Midway Island station is then advanced this number of nautical miles along a line parallel with the course line (although parallel rulers would be a big help, plotting is all done with dividers or compass and a small plotter.) The original DME distance is then swung from this advanced station using a double arrowheaded arc. This results in an advanced LOP from Midway Island, as shown (see diagram on page 39).

I am certain that rapid advances in technology will probably soon eliminate the pages in our flight operations manual devoted to running LOPs. It wasn't long ago that this same manual required us to plot the position of any ships spotted on long over-water legs. This was to be used in the event of a serious emergency involving ditching. It was a holdover from the days when the Pacific crossings were flown at 8,000 to 10,000 feet above the water (unlike today's altitudes of up to 41,000 feet), and when planes' piston engines lost power on a regular basis.

Northwest replaced the remaining Delco Carousel INS units with ring-laser gyro Litton INS units at the end of last year. However, the new training received to operate new equipment still requires the pilots to plot and verify the positions just as before.